Apparatus for, and method of, reducing noise in a communications system

ABSTRACT

A communication line having a plurality of twisted wire pairs connects a plurality of transmitters, one transmitter at each end of each twisted wire pair, with a plurality of receivers, one receiver at each end of each twisted wire pair. Each receiver receives a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the remaining transmitters at the opposite end of the communications line. A plurality of FEXT cancellation systems, one associated with each receiver, provides a replica FEXT impairment signal. A device associated with each receiver is responsive to the combination signal received by the receiver and the replica FEXT impairment signal provided by the FEXT cancellation system associated with the receiver for substantially removing the FEXT impairment signals from the combination signal. If necessary, a skew adjuster delays the arrival of the combination signal at the device so that the combination signal and the FEXT impairment signal arrive at the device at substantially the same time. A sequential decoder operates on signals from each of the plurality of wire pairs simultaneously to produce receiver outputs. A plurality of near-end crosstalk (NEXT) cancellation systems and echo cancellers remove NEXT and echo impairment signals from the combination signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to systems for, and methods of, providingfor the transmission and reception of signals through unshielded twistedpairs of wires within a communications system. The inventionparticularly relates to systems for, and methods of, reducing the noisepresent in the signals received and processed by devices within acommunications system and to systems for, and methods of, reducing suchnoise in communications systems having high throughputs. A “highthroughput” as used within the context of this disclosure may include,but is not limited to, one gigabit (GB) per second.

[0003] 2. Description of Related Art

[0004] A basic communications system is illustrated in FIG. 1. Thesystem includes a hub and a plurality of computers serviced by the hubin a local area network (LAN). Four computers are shown by way ofillustration but a different number of computers may be contained withinthe system. Each of the computers is usually displaced from the hub by adistance which may be as great as approximately one hundred meters (100m.). The computers are also displaced from each other. The hub isconnected to each of the computers by a communications line. Eachcommunication line includes unshielded twisted pairs of wires or cables.Generally, the wires or cables are formed from copper. Four unshieldedtwisted pairs of wires are provided in each communication line betweeneach computer and the hub. The system shown in FIG. 1 is operative withseveral categories of unshielded twisted pairs of cables designated ascategories 3, 4 and 5 in the telecommunications industry. Category 3cables are the poorest quality (and lowest cost) and category 5 cablesare the best quality (and highest cost).

[0005] Associated with each communications system is a “throughput”. Thethroughput of a system is the rate at which the system processes dataand is usually expressed in bits/second. Most communications systemshave throughputs of 10 megabits (Mb)/second or 100 Mb/second. A rapidlyevolving area of communications system technology enables 1 Gb/secondfull-duplex communication over existing category-5 unshielded twistedpair cables. Such a system is commonly referred to as “GigabitEthernet.”

[0006] A portion of a typical Gigabit Ethernet is shown in FIG. 2. TheGigabit Ethernet provides for transmission of digital signals betweenone of the computers and the hub and the reception of such signals atthe other of the computer and the hub. A similar system can be providedfor each of the computers The system includes a gigabit mediumindependent interface (GMII) block which receives data in byte-wideformat at a specified rate, for example 125 MHz, and passes the dataonto the physical coding sublayer (PCS) which performs scrambling,coding, and a variety of control functions. The PCS encodes bits fromthe GMII into 5-level pulse amplitude modulation (PAM) signals. The fivesignal levels are −2, −1, 0, +1, and +2. Communication between thecomputer and hub is achieved using four unshielded twisted pairs ofwires or cables, each operating at 250 Mb/second, and eighttransceivers, one positioned at each end of a unshielded twisted pair.The necessity of full-duplex bidirectional operation dictates the use ofhybrid circuits at the two ends of each unshielded twisted pair. Thehybrid controls access to the communication line, thereby allowing forfull-duplex bidirectional operation between the transceivers at each endof the communications line.

[0007] A common problem associated with communications systems employingmultiple unshielded twisted pairs and multiple transceivers is theintroduction of crosstalk and echo noise or impairment signals into thetransmission signals. Noise is inherent in all such communicationssystems regardless of the system throughput. However, the effects ofthese impairment signals are magnified in Gigabit Ethernet. Impairmentsignals include echo, near-end crosstalk (NEXT), and far-end crosstalk(FEXT) signals. As a result of these impairment signals the performanceof the transceivers, particularly the receiver portion, is degraded.

[0008] NEXT is an impairment signal that results from capacitivecoupling of the signals from the near-end transmitters to the input ofthe receivers. The NEXT impairment signals encountered by the receiverin transceiver A are shown in FIG. 3. The crosstalk signals fromtransmitters B, C, and D appears as noise to receiver A, which isattempting to detect the direct signal from transmitter E. Each of thereceivers in the system encounter the same effect and accordingly thesignals passing through the receivers experience signal distortion dueto NEXT impairment signals. For clarity of FIG. 3, only the NEXTimpairment experienced by receiver A is illustrated.

[0009] Similarly, because of the bidirectional nature of thecommunications systems, an echo impairment signal is produced by eachtransmitter on the receiver contained within the same transceiver as thetransmitter. The echo impairment signal encountered by the receiver ineach transceiver is shown in FIG. 4. The crosstalk signals fromtransmitters appear as noise to the receivers, which are attempting todetect the signal from the transmitter at the opposite end of thecommunications line. Each of the receivers in the system encounter thesame effect and accordingly the signals passing through the receiversexperience signal distortion due to the echo impairment signal.

[0010] Far-end crosstalk (FEXT) is an impairment that results fromcapacitive coupling of the signal from the far-end transmitters to theinput of the receivers. The FEXT impairment signals encountered by thereceiver in transceiver A are shown in FIG. 5. The crosstalk signalsfrom transmitters F, G, and H appears as noise to receiver A, which isattempting to detect the direct signal from transmitter E. Each of thereceivers in the system encounter the same effect and accordingly thesignals passing through the receivers experience signal distortion dueto the FEXT impairment signal. For clarity of FIG. 5 only the FEXTimpairment experienced by receiver A is illustrated.

[0011] As a result of these noise impairment signals the performance ofthe communication system is degraded. The signals carried by the systemare distorted and the system experiences a higher signal error rate.Thus there exists a need in the art to provide a method of, and anapparatus for, compensating for the degradation of communication systemperformance caused by noise impairment signals and to provide a methodof, and apparatus for, reducing such noise in a high throughput systemsuch a Gigabit Ethernet. The present invention fulfills these needs.

SUMMARY OF THE INVENTION

[0012] Briefly, and in general terms, the invention relates to systemsfor, and methods of, reducing the noise present in the signals receivedand processed by devices within a communications system and to systemsfor, and methods of, reducing such noise in communications systemshaving high throughputs.

[0013] In one embodiment, the invention is a communications systemincluding a communication line having a plurality of twisted wire pairsand a plurality of transmitters, one transmitter at each end of eachtwisted wire pair. The system also includes a plurality of receivers,one receiver at each end of each twisted wire pair. Each receiverreceives a combination signal including a direct signal from thetransmitter at the opposite end of the twisted wire pair with which thereceiver is associated and a plurality of far-end crosstalk (FEXT)impairment signals, one from each of the remaining transmitters at theopposite end of the communications line. Further included in the systemare a plurality of FEXT cancellation systems, one associated with eachreceiver; each FEXT cancellation system provides a replica FEXTimpairment signal. The system also includes a plurality of devices, oneassociated with each receiver; each device is responsive to thecombination signal received by the receiver and the replica FEXTimpairment signal provided by the FEXT cancellation system associatedwith the receiver for substantially removing the FEXT impairment signalsfrom the combination signal.

[0014] By providing a plurality of FEXT cancellation devices whichgenerate replica FEXT impairment signals and a plurality of device whichcombine the replica FEXT impairment signals with the combinationsignals, the invention substantially cancels the FEXT impairment signalsfrom the combination signal. Thus signal degradation due to noise in thecommunications system is reduced and the transmitted information may bemore reliably recovered.

[0015] In another embodiment, the invention is a method for reducingnoise in a communications system. The communications system comprises acommunication line having a plurality of twisted wire pairs, a pluralityof transmitters, one transmitter at each end of each of the twisted wirepairs, a plurality of receivers, one receiver at each end of each of thetwisted wire pairs. Each receiver receives a combination signalincluding a direct signal from the transmitter at the opposite end ofthe twisted wire pair with which the receiver is associated and aplurality of far-end crosstalk (FEXT) impairment signals, one from eachof the remaining transmitters at the opposite end of the communicationsline. The communications system further comprises a plurality ofcombining devices, one associated with each receiver. The methodcomprises the step of generating a replica FEXT impairment signal. Alsoincluded is the step of delaying the arrival of the combination signal,at the combining device, by an amount substantially equal to the timedelay between the arrival, at the receiver, of the direct signal and theFEXT impairment signals. Further included is the step of subtracting thereplica FEXT impairment signal from the combination signal to produce anoutput signal substantially devoid of FEXT impairment signals.

[0016] In yet another embodiment, the invention is a system comprising acommunication line having a plurality of twisted wire pairs and having atransmitter and a receiver connected at each end of each twisted wirepair to transmit and receive sequences of signals. The system alsoincludes a decoder responsive to the sequences of the signals from thereceivers at the same ends of the twisted wire pairs for determiningwhether such sequences of signals are acceptable and for processing suchsequences of signals in accordance with such determinations.

[0017] In an additional facet of this embodiment of the invention thedecoder determines whether the sequences of the signals at the receiversare acceptable or unacceptable based on a standard system code. The codeintroduces constraints in the sequences of symbols that can betransmitted in such a way as to increase the minimum distance betweentwo arbitrary allowable sequences. This increased minimum distanceresults in a reduced probability of error. The decoder passes acceptablesequences of signals and rejects unacceptable sequences. Acceptablesequences are those sequences which obey the code constraints whileunacceptable sequences are those which violate the code constraints.

[0018] These and other aspects and advantages of the present inventionwill become apparent from the following more detailed description, whentaken in conjunction with the accompanying drawings which illustrate, byway of example, the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic block diagram of a communications systemproviding a plurality of computers connected to a hub by communicationslines to form a local area network (LAN);

[0020]FIG. 2 is a schematic block diagram of a communications systemproviding a gigabit medium independent interface (GMII), a physicalcoding sublayer (PCS) and a plurality of unshielded twisted pairs ofwires, each with a transceiver at each end;

[0021]FIG. 3 is a schematic block diagram of a portion of thecommunications system of FIG. 2 depicting the NEXT impairment signalsreceived by receiver A from adjacent transmitters B, C, and D;

[0022]FIG. 4 is a schematic block diagram of a portion of thecommunications system of FIG. 2 depicting the echo impairment signalreceived by receiver A from transmitter A;

[0023]FIG. 5 is a schematic block diagram of a portion of thecommunications system of FIG. 2 depicting the FEXT impairment signalsreceived by receiver A from opposite transmitters F, G, and H;

[0024]FIG. 6 is a schematic block diagram of a communications system inaccordance with one embodiment of the present invention including aplurality of transceivers each having a NEXT cancellation system, anecho canceller, and a FEXT cancellation system, digital adaptive filtersystem including a plurality of detectors and a skew adjuster, and atiming recovery circuit;

[0025]FIG. 7 is a schematic block diagram of a symbol-by-symbol detectorof FIG. 6, each including a plurality of slicers, feedback filters andadders and receiving as input a soft decision;

[0026]FIG. 8 is a schematic block diagram of the NEXT cancellationsystems of FIG. 6, each including a plurality of adaptive transversalfilters (ATF) and adders and receiving as input transmitted signals fromadjacent transmitters;

[0027]FIG. 9 is a schematic block diagram of the echo cancellers of FIG.6, each including an ATF and receiving as input transmitted signals fromsame transmitters;

[0028]FIG. 10 is a schematic block diagram of the FEXT cancellationsystems of FIG. 6, each including a plurality of ATFs and an adder andreceiving as input transmitted signals from opposite transmitters;

[0029]FIG. 11 depicts a direct impulse response arriving at the receiverafter a FEXT impulse response;

[0030]FIG. 12 depicts a direct impulse response and FEXT impulseresponse arriving at the receiver at substantially the same time;

[0031]FIG. 13 depicts a direct impulse response arriving at the receiverbefore a FEXT impulse response; and

[0032]FIG. 14 is a schematic block diagram of a communications system inaccordance with one embodiment of the present invention including aplurality of transceivers, each having a NEXT cancellation system, anecho canceller, and a FEXT cancellation system, digital adaptive filtersystem including one detector, and a timing recovery circuit

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The discussion in this specification may be considered to relatespecifically to a Gigabit Ethernet for the purposes of explanation andunderstanding of the invention. However, it will be understood that theconcepts of this invention and the scope of the claims apply to othertypes of communications systems other than a Gigabit Ethernet.

[0034] A communications system incorporating the features of thisinvention is generally indicated at 10 in FIG. 1. The system 10 includesa hub 12 and a plurality of computers serviced by the hub in a localarea network (LAN). Four computers 14 are shown by way of illustrationbut a different number of computers may be used without departing fromthe scope of the invention. Each of the computers 14 may be displacedfrom the hub 12 by a distance as great as approximately one hundredmeters (100 m.). The computers 14 are also displaced from each other.

[0035] The hub 12 is connected to each of the computers 14 by acommunications line 16. The communication line 16 comprises a pluralityof unshielded twisted pairs of wires or cables. Generally, the wires orcables are formed from copper. Four unshielded twisted pairs of wiresare provided in the system 10 between each computer and the hub 12. Thesystem shown in FIG. 1 is operative with several categories of twistedpairs of cables designated as categories 3, 4 and 5 in thetelecommunications industry. Category 3 cables are the poorest quality(and lowest cost) and category 5 cables are the best quality (andhighest cost). Gigabit Ethernet uses category 5 cables.

[0036]FIG. 2 illustrates, in detail, a portion of the communicationssystem of FIG. 1 including one communications line 16 and portions ofone of the computers 14 and the hub 12. The communications line 16includes four unshielded twisted pairs of wires 18 operating at 250Mb/second per pair. A transceiver 20, including a transmitter (TX) 22and receiver (RX) 24, is positioned at each unshielded end of eachtwisted pair 18. Between each transceiver 20 and its associatedunshielded twisted pair 18 is a hybrid 26. The hybrid 26 controls accessto the communication line 16, thereby allowing for full-duplexbidirectional operation between the transceivers 20 at each end of thecommunications line. The hybrid also functions to isolate thetransmitter and receiver associated with the transceiver, from eachother.

[0037] The communications system includes a standard connectordesignated as a gigabit media independent interface (GMII). The GMII maybe an eight bit wide data path in both the transmit and receivedirections. Clocked at a suitable frequency, such as 125 MHz, the GMIIresults in a net throughput in both directions of data at a suitablerate such as 250 Mb/second per pair. The GMII provides a symmetricalinterface in both the transmit and receive directions. A physical codingsublayer (PCS) 30 receives and transmits data between the GMII 28 andthe transceivers 20. The PCS 30 performs such functions as scramblingand encoding/decoding data before forwarding the data to either thetransceiver or the GMII. The PCS encodes bits from the GMII into 5-levelpulse amplitude modulation (PAM) signals. The five signal levels are −2,−1, 0, +1, and +2. The PCS also controls several functions of thetransceivers, such as skew control as explained below.

[0038] Four of the transceivers 20 are illustrated in detail in FIG. 6.The components of the transceivers 20 are shown as overlapping blocks,with each layer corresponding to one of the transceivers. The GMII 28,PCS 30, and hybrid 26 of FIG. 6 correspond to the GMII, PCS, and hybridof FIG. 2 and are considered to be separate from the transceiver. Thecombination of the transceiver 20 and hybrid 26 forms one “channel” ofthe communications system. Accordingly, FIG. 6 illustrates fourchannels, each of which operate in a similar manner. The transmitterportion of each transceiver 20 includes a pulse shaping filter 32 and adigital to analog (D/A) converter 34. In a preferred embodiment of theinvention the D/A converter 34 operates at 125 MHz. The pulse shapingfilter 32 receives one one-dimensional (1-D) symbol from the PCS. Thissymbol is referred to as a TXDatax symbol 36, where x is 1 through 4corresponding to each of the four channels. The TXDatax symbol 36represents 2 bits of data. The PCS generates one 1-D symbol for each ofthe channels. The symbol for each channel goes through a spectrumshaping filter of the form 0.75+0.25 z⁻¹ at the pulse shaping filter 32to limit emissions within FCC requirements. This simple filter shapesthe spectrum at the output of the transmitter so that its power spectraldensity falls under that of communications systems operating at 100Mb/second on two pairs of category-5 twisted pair wires. The symbol isthen converted into an analog signal by the D/A converter 34 which alsoacts as a lowpass filter. The analog signal gains access to theunshielded twisted pair wire 18 through the hybrid circuitry 26.

[0039] The receiver portion of each transceiver includes an A/Dconverter 42, a FIFO 44, a digital adaptive equalizer system, a timingrecovery circuit and noise reduction circuitry. The digital adaptiveequalizer system includes a feed-forward equalizer (FFE) 46, two devices50, 56, a skew adjuster 54 and two detectors 58, 60. The functions ofthese components, as related to the present invention, are explainedbelow. The general concept of the use of a digital adaptive equalizer ina communications system is disclosed in U.S. Pat. No. 5,604,741 toSamueli et al. entitled ETHERNET SYSTEM. The noise reduction circuitryincludes a NEXT cancellation system 38, an echo canceller 40, and a FEXTcancellation system 70.

[0040] The A/D converter 42 provides digital conversions of the signalsreceived from the hybrid 26 at a suitable frequency, such as 125 MHz,which is equal to the baud rate of the signals. The A/D converter 42samples the analog signals in accordance with an analog sample clocksignal 78 provided by the decision-directed timing recovery circuit 64.The FIFO 44 receives the digital conversion signals from the A/Dconverter 42 and stores them on a first-in-first-out basis. The FIFOforwards individual signals to the FFE 46 in accordance with a digitalsample clock signal 80 provided by the timing recovery circuit 64. Thefeed forward equalizer (FFE) 46 receives digital signals from the FIFOand filters these signals. The FFE 46 is a least mean squares (LMS) typeadaptive filter which performs channel equalization and precursor intersymbol interference (ISI) cancellation to correct for distortions in thesignal.

[0041] It should be noted that the signal introduced into the A/Dconverter 42 and subsequently into the FIFO 44 and FFE 46 has severalcomponents. These components include the direct signal received directlyfrom the transmitter 22 at the opposite end of the unshielded twistedpair wire 18 with which the receiver 24 is associated. Also included areone or more of the NEXT, echo, and FEXT impairment signals from othertransmitters 22 as previously described. The signal including the directsignal and one or more of the impairment signals is referred to as a“combination signal.”

[0042] The FFE 46 forwards the combination signal 48 to a second device50, typically a summing device. At the second device 50 the combinationsignal 48 is combined with the outputs of the NEXT cancellation system38 and echo canceller 40 to produce a signal which is substantiallydevoid of NEXT and echo impairment signals. This signal is referred toas a “first soft decision” 52. The skew adjuster 54 receives the firstsoft decision 52 from the second device 50 and outputs a signal referredto as a “second soft decision” 66″. The skew adjuster 54 performs twofunctions. First, it compensates for the difference in length of theunshielded twisted pairs 18 by delaying the first soft decision 52 sothat the second soft decisions 66 from all of the receivers in thesystem are in sync. Second, it adjusts the delay of the first softdecision 52 so that the second soft decision 66 arrives at the firstdevice 56 at substantially the same time as the output of the FEXTcancellation system 70. The skew adjuster 54 receives skew controlsignals 82 from the PCS 30.

[0043] The skew adjuster 54 forwards the second soft decision 66 to afirst device 56, typically a summing device. At the first device 56 thesecond soft decision 66 is combined with the output of the FEXTcancellation system 70 to produce a signal which is substantially devoidof FEXT impairment signals. This signal is referred to as a “third softdecision” 68. The first detector 58 receives the third soft decision 68from the first device 56. The first detector 58 provides an outputsignal, i.e., a “final decision” 72. The detector may be a slicer whichproduces a final decision 72 corresponding to the analog signal levelclosest in magnitude to the level of the third soft decision 68. Thedetector may also be either a symbol-by-symbol detector or a sequentialdetector which operates on sequences of signals across all four channelssimultaneously, such as a Viterbi decoder.

[0044] In a preferred embodiment of the invention the first detector 58is a symbol-by-symbol detector. A group of symbol-by-symbol detectors58, one for each channel, is shown in FIG. 7. Each first detector 58includes a slicer 98, adaptive feedback filter 100 and an adder 102. Theadder 102 combines the third soft decision 68 with the output of theadaptive feedback filter 100 to provide an output which is introduced tothe slicer 98. The output of the slicer 98 in introduced to the adaptivefeedback filter 100. The first detector 58 provides an output signal 72which corresponds to the discrete level from the set [−2, −1, 0, 1, 2]which is closest to the difference between the third soft decision 68and the output of the feedback filter 100. The adaptive feedback filter100 corrects for distortion in the third soft decision 68. This filter100 uses past slicer 98 decisions to estimate postcursor ISI caused bythe channel. This ISI is canceled from the third soft decision 68 toform the final decision signal 72.

[0045] In another embodiment of the invention the first detector 58 is acombination of a sequential decoder with a decision feedback equalizer(DFE) using the architecture usually known as multiple DFE architecture(MDFE) sequential detector. The sequential decoder 58 looks at allsignals from all four channels at the same time and at successivesamples from each channel over several periods of unit time. Asequential decoder receives as input at least one signal from each ofthe first devices 56. The sequential decoder 58, in general, isresponsive to the sequences of the output signals from the first devices56 for (1) passing acceptable sequences of such signals and (2)discarding unacceptable sequences of such signals in accordance with theconstraints established by the code standard associated with the system.Acceptable sequences are those which obey the code constraints andunacceptable sequences are those which violate the code constraints.

[0046] The second detector 60 (FIG. 6) receives the first soft decision52 from the second device 50. The second detector 60 is asymbol-by-symbol detector. It provides an output signal 74 whichcorresponds to the discrete level from the set [−2, −1, 0, 1, 2] whichis closest to the difference between the first soft decision 52 and theoutput of the feedback filter 100. The second detector 60 producesoutput signals 74 without the benefit of FEXT cancellation, as a result,these decisions have a higher error rate than those made by the firstdetector 58, which enjoys the benefits of FEXT cancellation. Because ofthis fact, these decisions are called “tentative decisions”. It isimportant to note that the postcursor ISI present in the input to thesecond detector 60 is canceled using the auxiliary feedback filter 100,(FIG. 7) contained within the second detector, whose inputs are thetentative decisions 74. The coefficients of this adaptive feedbackfilter 100 are the same as those of the adaptive feedback filterassociated with the first detector 58 (FIG. 6).

[0047] A third device 62, typically a summing device, receives the firstsoft decision signal 52 from the second device 50 and the tentativedecision signals 74 from the second detector 60. At the third device 62the first soft decision 52 is combined with the tentative decisionsignal 74 to produce an error signal 76 which is introduced into thetiming recovery circuit 64. The timing recovery circuit 64 receives thetentative decision 74 from the second detector 60 and the error signals76 from the third device 62. Using these signals as inputs the timingrecovery circuit 64 outputs an analog clock sync signal 78 which isintroduced to the A/D converter 42 and a digital clock sync signal 80which is introduced into the FIFO 44. As previously mentioned, thesesignals control the rate at which the A/D converter 42 samples theanalog input it receives from the hybrid 26 and the rate at which theFIFO forwards digital signals to the FFE 46. A suitable timing recoverydevice for use in the present invention is disclosed in copending patentapplication Ser. No. 08/970,557 entitled APPARATUS FOR, AND METHOD OF,PROCESSING SIGNALS TRANSMITTED OVER A LOCAL AREA NETWORK and assigned ofrecord to the assignee of record of this application.

[0048] As mentioned before, the symbols sent by the transmitters 22(FIG. 2) in the communications system cause NEXT, echo and FEXTimpairments in the received signal for each channel. Since each receiver24 has access to the data for the other three channels that cause thisinterference, it is possible to nearly cancel each of these effects.NEXT cancellation is accomplished using three adaptive NEXT cancellingfilters as shown in the block diagram of FIG. 8. Each NEXT cancellationsystem 38 receives three TXDatax symbols 36 from each of thetransmitters at the same end of the communications line 18 as thereceiver with which the NEXT cancellation system is associated. EachNEXT cancellation system 38 includes three filters 84, one for each ofthe TXDatax symbols 36. These filters 84 model the impulse responses ofthe NEXT noise from the transmitters and may be implemented as adaptivetransversal filters (ATF) employing, for example, the LMS algorithm. Thefilters 84 produce a replica of the NEXT impairment signal for eachTXDatax symbol 36. A summing device 86 combines the three individualreplica NEXT impairment signals 92 to produce a replica of the NEXTimpairment signal contained within the combination signal received bythe receiver with which the NEXT cancellation system 38 is associated.The replica NEXT impairment signal 88 is introduced into the seconddevice 50 (FIG. 6) where it is combined with the combination signal 48to produce a first soft decision signal 52 which is substantially devoidof NEXT impairment signals.

[0049] Echo cancellation is accomplished with an adaptive cancellingfilter 84 as shown in the block diagram of FIG. 9. Each echo canceller40 receives the TXDatax symbols 36 from the transmitter at the same endof the twisted wire pair 18 as that of the receiver with which the echocanceller is associated. As shown in FIG. 9, each echo canceller 40includes one filter 84. These filters 84 model the impulse responses ofthe echo noise from the transmitter and may be implemented as ATFsemploying, for example, the LMS algorithm. The filter produces a replicaof the echo impairment signal contained within the combination signalreceived by the receiver with which the echo canceller 40 is associated.The replica echo impairment signal 90 is introduced into the seconddevice 50 (FIG. 6) where it is combined with the combination signal 48to produce the first soft decision signal 52 which is substantiallydevoid of echo impairment signals.

[0050] FEXT cancellation is accomplished with three adaptive FEXTcancelling filters 84 as shown in the block diagram of FIG. 10. EachFEXT cancellation system 70 receives three tentative decision symbols 74one from each of the receivers at the same end of the communicationsline as the receiver with which the FEXT cancellation system isassociated. Each FEXT cancellation system 70 includes three filters 84,one for each of the tentative decision symbols 74. These filters 84model the impulse responses of the FEXT noise from transmitters and maybe implemented as ATFs employing, for example, the LMS algorithm. Thefilters 84 produce a replica of the FEXT impairment signal 96 for eachindividual tentative decision symbol 74. A summing device 108 combinesthe three individual replica FEXT impairment signals 96 to produce areplica of the FEXT impairment signal contained within the combinationsignal 48 received by the receiver with which the FEXT cancellationsystem is associated. The replica FEXT impairment signal 94 isintroduced into the first device 56 (FIG. 6) where it is combined withthe second combination signal 66 to produce the third soft decisionsignal 68 which is substantially devoid of FEXT impairment signals. Itis important to note that the higher error rate of the tentativedecisions 74 does not degrade the performance of the FEXT cancellationsystem 70, because the decisions used to cancel FEXT are statisticallyindependent from the final decisions 72 made by the receiver whose FEXTis being canceled.

[0051] The symbols provided by the first detector 58 are decoded anddescrambled by the receive section of the PCS 30 before being introducedto the GMII. Variations in the way the wire pairs are twisted may causedelays through the four channels by up to 50 nanoseconds. As a result,the symbols across the four channels may be out of sync. As previouslymentioned, in the case where the first detector is a sequentialdetector, the PCS also determines the relative skew of the four streamsof 1-D symbols and adjusts the symbol delay, through the skew adjuster54, prior to their arrival at the first detector 58 so that sequentialdecoder can operate on properly composed four-dimensional (4-D) symbols.Additionally, since the cabling plant may introduce wire swaps within apair and pair swaps among the four unshielded twisted pairs, the PCS 30also determines and corrects for these conditions.

[0052] As previously mentioned, FEXT is an impairment that results fromcapacitive coupling of the signal from the far-end transmitters to theinput of the receivers, as shown in FIG. 5. The crosstalk signals fromtransmitters F, G, and H appear as noise to receiver A, which isattempting to detect the signal from transmitter E. A similar situationapplies to all other receivers regarding the signals from theappropriate transmitters located at the opposite end of the line.

[0053] The FEXT noise experienced by receiver A and originating fromtransmitter F can be modeled as the convolution of the data symbolstransmitted by F with a certain impulse response that depends on theproperties of the cable and models the coupling characteristics of theunshielded twisted pairs used by transmitter F and receiver A. A typicalmeasured FEXT impulse response 104 is show in FIGS. 11-13. A similardescription can be given for all the other possible receiver-transmittercombinations. Therefore, there are a total of twelve FEXT impulseresponses 104 describing the FEXT noise signals from transmitters E, F,G, and H to receivers A, B, C, and D. These twelve impulse responses arenot identical, although each has a general shape similar to that shownin FIGS. 11-13.

[0054] Although FEXT is an impairment for many communications systemsother than Gigabit Ethernet, in these systems a given receiver usuallydoes not have access to the symbols detected by the other receivers,because these receivers may not be physically located in the same place,and/or because they operate at rates that are not synchronized to thedata rate of the receiver suffering from FEXT. The present inventiontakes advantage of the fact that in Gigabit Ethernet transceivers thedecisions that correspond to all four channels are available to the fourreceivers and the decisions may be made synchronous.

[0055] In operation there may be delays associated with the transmissionof signals across the communications line. The synchronization of thesignals within the system is crucial to effective cancellation of thenoise. It is important that the replica noise impairment signals arriveat the summing devices at substantially the same time as the combinationsignal and/or soft decision signals. With regard to the FEXT impairmentsignal, because the impairment is caused by the transmitters at theopposite end of the receiver there is likely to be a delay between thetime that the second soft decision signal 66 arrives at the first device56 and the time at which the replica FEXT impairment signal 94 arrives.In some channels, as illustrated in FIG. 11, the group delay of the FEXTsignal 104 could be smaller than the group delay of the desired signal106. In this case the tentative decisions 74 provided by receivers B, C,and D of FIG. 5 arrive at the FEXT cancellation system 70 of receiver Atoo late to be effective in canceling the FEXT impairment.

[0056] To compensate for this delay, the invention employs a skewadjuster 54 which, as previously stated, delays the first soft decisionsignal 52 by a time substantially equal to or greater than the timedelay between the arrival at the receiver of the direct signal and theFEXT impairment signals associated with such receiver. If the output isdelayed by an amount greater than the time delay, which would result inthe situation illustrated in FIG. 13, the adaptive feedback filter 84(FIG. 10) within the FEXT cancellation system 70 compensates for theover delay by delaying the replica FEXT impairment signal 94 so that itarrives at the first device 56 (FIG. 6) at substantially the same timeas the second soft decision signal 66.

[0057] The third soft decision 68 resulting from FEXT cancellationallows the first detector 58 to make more reliable final decisions 72,with a greatly reduced error rate. Computer simulations show that atypical improvement achievable with the invention described herein isapproximately 2 to 3 dB when the signal to noise ratio at the input ofthe first detector 58 before FEXT cancellation is approximately 25 dB.This corresponds to a reduction of the symbol error rate of a factor1000 or larger.

[0058] If there is no delay associated with the transmission of signalsacross the communications line both the FEXT impairment signal 104 andthe direct signal 106 arrive at the receiver at the substantially thesame time, as shown in FIG. 12. In this situation, the delay of the skewadjuster 54 is set to zero. In the alternative, an embodiment of theinvention, as shown in FIG. 14, with only one detector 110 and onesumming device 112 may be used. In this configuration, the summingdevice 112 receives the replica NEXT, echo, and FEXT impairment signals88, 90, 94 and the combination signal 48 and produces a first softdecision 52 substantially devoid of impairment signals. This first softdecision 52 is introduced into the detector 110 and the third device 62.The detector 110 may include either a single symbol-by-symbol detectoror both a symbol-by-symbol detector and a sequential detector. In thecase of a symbol-by-symbol detector the final decision 72 and secondoutput 114 of the detector 110 are identical. In the case of both asymbol-by-symbol detector and a sequential detector, the final output isprovided by the sequential detector and is introduced to the PCS 30. Thesecond output 114 is provided by the symbol-by-symbol detector and isintroduced to the timing recovery circuitry 64 and the third device 62for use in determining the error signal 76.

[0059] Although this invention has been disclosed and illustrated withreference to particular embodiments, the principles involved aresusceptible for use in numerous other embodiments which will be apparentto persons of ordinary skill in the art. The invention is, therefore, tobe limited only as indicated by the scope of the appended claims.

In the claims:
 1. A communications system comprising: a communicationline having a plurality of twisted wire pairs; a plurality oftransmitters, one transmitter at each end of each twisted wire pair; aplurality of receivers, one receiver at each end of each twisted wirepair, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of far-endcrosstalk (FEXT) impairment signals, one from each of the remainingtransmitters at the opposite end of the communications line; a pluralityof FEXT cancellation systems, one associated with each receiver, eachFEXT cancellation system for providing a replica FEXT impairment signal;and a plurality of devices, one associated with each receiver, eachdevice responsive to the combination signal received by such receiverand the replica FEXT impairment signal provided by the FEXT cancellationsystem associated with such receiver for substantially removing the FEXTimpairment signals from the combination signal.
 2. The communicationssystem of claim 1, wherein the FEXT cancellation system comprises: meansfor receiving a signal from each of the receivers at the same end of thecommunications line except for the receiver with which the FEXTcanceller is associated; means for generating an individual replica FEXTimpairment signal for each receiver signal; and means for combining theindividual replica FEXT impairment signals to generate the replica FEXTimpairment signal.
 3. The communications system of claim 2 furthercomprising a detector system responsive to the output signal of eachdevice for providing, for each device, a detector output signalcorresponding to the analog signal level closest in magnitude to thelevel of the device output signal.
 4. The communications system of claim3 wherein each device output signal is a soft decision signal comprisinga plurality of bits, said detector system comprising a symbol-by-symboldecoder responsive to the signals to provide both a final decisionsignal and one of the receiver signals to each of the FEXT cancellationsystems at the same end of the communications line, except for the FEXTcancellation system associated with such device.
 5. The communicationssystem of claim 4 further comprising a delay device for delaying thefinal decisions such that the final decisions are in sync with eachother.
 6. The communications system of claim 3 wherein each deviceoutput signal is a soft decision signal comprising a plurality of bits,said detector system comprising: a decoder which is responsive to thesignals on a sequential basis to provide final decision signals; and asymbol-by-symbol decoder which is responsive to the signals to provideone of the receiver signals to each of the FEXT cancellation systems atthe same end of the communications line, except for the FEXTcancellation system associated with such device.
 7. The communicationssystem of claim 6 further comprising a delay device for delaying thefinal decisions such that the final decisions are in sync with eachother.
 8. The communications system of claim 2 further comprising: aplurality of analog to digital (A/D) converters, one associated witheach receiver, each A/D converter responsive to the combination signalreceived by such receiver for providing a digital conversion of thecombination signal; a plurality of equalizer systems, one associatedwith each receiver, each equalizer system responsive to the digitalconversion combination signal for providing an adaptive equalization ofsuch digital conversion combination signal; and a plurality offirst-in-first-out (FIFO) devices, one associated with each receiver,each FIFO device receiving the output of the A/D converter andforwarding the output to the equalizer system on a FIFO basis.
 9. Thecommunications system of claim 8 further comprising a plurality ofhybrids, one hybrid at each end of each twisted wire pair, each hybridfor isolating the transmitter associated with such twisted wire pairfrom the receiver associated with such twisted wire pair, each hybridresponsive to the combination signal for forwarding the combinationsignal to the A/D converter.
 10. The communications system of claim 8further comprising: a detector system responsive to the output signal ofeach device for providing, for each device, a detector output signal;and a plurality of timing recovery devices, one associated with eachreceiver, each timing recovery device responsive to at least one of theoutputs of the detector system for regulating the rate of production ofthe A/D converter so that the digital conversions of the signalsreceived by the A/D converter are at a particular rate and in aparticular phase and for regulating the rate at which the FIFO forwardsthe output of the A/D converter to the equalizer system.
 11. Thecommunications system of claim 10 wherein the detector output signalcorresponds to the analog signal level closest in magnitude to the levelof the device output signal.
 12. The communications system of claim 10wherein the detector output signal corresponds to one of a discrete setof symbols.
 13. The communications system of claim 12 wherein thedetector system comprises a feedback filter, the discrete set of symbolsis [−2, −1, 0, 1, 2] and the symbol chosen is that symbol closest to thedifference between the device output signal and the output of thefeedback filter.
 14. The communications system of claim 10 wherein eachdevice output signal is a soft decision signal comprising a plurality ofbits, said detector comprising a symbol-by-symbol decoder responsive tothe signals to provide a final decision signal, one of the receiversignals to each of the FEXT cancellation systems at the same end of thecommunications line, except for the FEXT cancellation system associatedwith such device, and the input to the timing recovery device associatedwith such receiver.
 15. The communications system of claim 14 furthercomprising a delay device for delaying the final decisions such that thefinal decisions are in sync with each other.
 16. The communicationssystem of claim 10 wherein each device output signal is a soft decisionsignal comprising a plurality of bits, said detector system comprising:a decoder which is responsive to the signals on a sequential basis toprovide final decision signals; and a symbol-by symbol decoder which isresponsive to the signals to provide one of the receiver signals to eachof the FEXT cancellation systems at the same end of the communicationsline, except for the FEXT cancellation system associated with suchdevice and the input to the timing recovery device associated with suchreceiver.
 17. The communications system of claim 16 further comprising adelay device for delaying the final decisions such that the finaldecisions are in sync with each other.
 18. The communications system ofclaim 1 wherein the combination signal further includes a plurality ofnear-end crosstalk (NEXT) impairment signals, one from each of thetransmitters at the same end of the communications line, except for thetransmitter at the same end of the twisted wire pair with which thereceiver is associated, said communications system further comprising aplurality of NEXT cancellation systems, one associated with eachreceiver, each NEXT cancellation system for providing a replica NEXTimpairment signal, wherein each said device is further responsive to thereplica NEXT impairment signal for substantially canceling the pluralityof NEXT impairment signals from the combination signal.
 19. Thecommunications system of claim 1 wherein the combination signal furtherincludes an echo impairment signal received from the transmitter at thesame end of the twisted wire pair with which the receiver is associated,said communications system further comprising a plurality of echocancellers, one associated with each of receiver, each echo cancellerfor providing a replica echo impairment signal, wherein each said deviceis further responsive to the replica echo impairment signal forsubstantially canceling the echo impairment signal from the combinationsignal.
 20. A communications system comprising: a communication linehaving a plurality of twisted wire pairs; a plurality of transmitters,one transmitter at each end of each twisted wire pair; a plurality ofreceivers, one receiver at each end of each twisted wire pair, eachreceiver receiving a combination signal including a direct signal fromthe transmitter at the opposite end of the twisted wire pair with whichthe receiver is associated and a plurality of far-end crosstalk NEXTimpairment signals, one from each of the remaining transmitters at theopposite end of the communications line; a plurality of FEXTcancellation systems, one associated with each receiver, each FEXTcancellation system for providing a replica FEXT impairment signal; aplurality of delay devices, one associated with each receiver, eachdelay device responsive to the combination signal received by suchreceiver for delaying the combination signal; and a plurality of firstdevices, one associated with each receiver, each first device responsiveto the output of the delay device associated with such receiver and thereplica FEXT impairment signal provided by the FEXT cancellation systemassociated with such receiver for substantially removing the FEXTimpairment signals from the combination signal.
 21. The communicationssystem of claim 20, wherein the FEXT cancellation system comprises:means for receiving a signal from each of the receivers at the same endof the communications line except for the receiver with which the FEXTcanceller is associated; means for generating an individual replica FEXTimpairment signal for each received signal; and means for combining theindividual replica FEXT impairment signals to generate the replica FEXTimpairment signal.
 22. The communications system of claim 21 wherein,when the direct signal arrives at the receiver after the FEXT impairmentsignals, the delay device delays the combination signal by an amountsubstantially equal to the time delay between the arrival, at thereceiver, of the FEXT impairment signals and the direct signal.
 23. Thecommunications system of claim 21 wherein the delay device delays thecombination signal so that such combination signal is in sync with thecombination signals from other receivers.
 24. The communications systemof claim 21 wherein, when the direct signal arrives at the receiverafter the FEXT impairment signals, the delay device delays thecombination signal by the greater of the following: an amountsubstantially equal to the time delay between the arrival, at thereceiver, of the FEXT impairment signals and the direct signal; and anamount so that such combination signal is in sync with the combinationsignals from other receivers.
 25. The communications system of claim 21wherein, when the direct signal arrives at the receiver after the FEXTimpairment signals, the delay device delays the combination signal by anamount greater than the time delay between the arrival, at the receiver,of the FEXT impairment signals and the direct signal and the FEXTcancellation system, in turn, delays the replica FEXT impairment signalby an amount substantially equal to the greater amount.
 26. Thecommunications system of claim 21 wherein, when the FEXT impairmentsignals arrive at the receiver after the direct signal, the FEXTcancellation system delays the replica FEXT impairment signal by anamount substantially equal to the time delay between the arrival, at thereceiver, of the direct signal and the FEXT impairment signals.
 27. Thecommunications system of claim 20 further comprising: a plurality ofanalog to digital (A/D) converters, one associated with each receiver,each A/D converter responsive to the combination signal received by suchreceiver for providing a digital conversion of the combination signal; aplurality of equalizer systems, one associated with each receiver, eachequalizer system responsive to the digital conversion combination signalfor providing an adaptive equalization of such digital conversioncombination signal; and a plurality of first-in-first-out (FIFO)devices, one associated with each receiver, each FIFO device receivingthe output of the A/D converter and forwarding the output to theequalizer system on a FIFO basis.
 28. The communications system of claim27 further comprising: a first detector responsive to the output signalof each first device for providing a first detector output signalcorresponding to the analog signal level closest in magnitude to thelevel of the first device output signal; and a second detectorresponsive to the output signal of each equalizer system for providing asecond detector output signal corresponding to the analog signal levelclosest in magnitude to the level of the equalizer output signal. 29.The communications system of claim 27 further comprising: a firstdetector responsive to the output signal of each first device forproviding a first detector output signal corresponding to one of adiscrete set of symbols; and a second detector responsive to the outputsignal of each equalizer system for providing a second detector outputsignal corresponding to one of the discrete set of symbols.
 30. Thecommunications system of claim 29 wherein the first and second detectorseach comprise a feedback filter, the discrete set of symbols is [−2, −1,0, 1, 2] and for the first detector, the symbol chosen is that symbolclosest to the difference between the first device output signal and theoutput of the first detector feedback filter and for the seconddetector, the symbol chosen is that symbol closest to the differencebetween the output of the equalizer system and the output of the seconddetector filter.
 31. The communications system of claim 28 wherein thefirst device output signal and the equalizer output signal are softdecision signals comprising a plurality of bits, the first detector andsecond detector are symbol-by-symbol decoders responsive to the signals,the first detector providing a final decision signal and the seconddetector providing a tentative decision signal for each of the FEXTcancellation systems at the same end of the communications line, exceptfor the FEXT cancellation system associated with such device.
 32. Thecommunications system of claim 28 wherein the first device output signaland the equalizer output signal are soft decision signals comprising aplurality of bits, the first detector is a decoder which is responsiveto the signals on a sequential basis to provide a final decision signaland the second detector is a symbol-by symbol decoder which isresponsive to the signals to provide a tentative decision signal foreach of the FEXT cancellation systems at the same end of thecommunications line, except for the FEXT cancellation system associatedwith such device.
 33. The communications system of claim 28 furthercomprising: a plurality of timing recovery devices, one associated witheach receiver, each timing recovery device responsive to at least one ofthe second detector outputs for regulating the rate of production of theA/D converter so that the digital conversions of the signals received bythe A/D converter are at a particular rate and in a particular phase andfor regulating the rate at which the FIFO forwards the output of the A/Dconverter to the equalizer system.
 34. The communications system ofclaim 33 further comprising: a plurality of error devices, oneassociated with each receiver, each error device responsive to theoutput of the equalizer system associated with such receiver and one ofthe second detector output signals for providing an error signal to thetiming recovery device associated with such receiver.
 35. Thecommunications system of claim 27 further comprising a plurality ofhybrids, one hybrid at each end of each twisted wire pair, each hybridfor isolating the transmitter associated with such twisted wire pairfrom the receiver associated with such twisted wire pair, each hybridresponsive to the combination signal for forwarding the combinationsignal to the A/D converter.
 36. The communications system of claim 28wherein the combination signal further includes a plurality of near-endcrosstalk (NEXT) impairment signals, one from each of the transmittersat the same end of the communications line, except for the transmitterat the same end of the twisted wire pair with which the receiver isassociated, said communications system further comprising: a pluralityof NEXT cancellation systems, one associated with each receiver, eachNEXT cancellation system for providing a replica NEXT impairment signal;a plurality of second devices, one associated with each receiver, eachsecond device responsive to the output of the equalizer systemassociated with such receiver and the replica NEXT impairment signalprovided by the NEXT cancellation system associated with such receiverfor substantially removing the NEXT impairment signals from thecombination signal and providing the signal to the delay device and thesecond detector.
 37. The communications system of claim 28 wherein thecombination signal further includes an echo impairment signal receivedfrom the transmitter at the same end of the twisted wire pair with whichthe receiver is associated, said communications system furthercomprising: a plurality of echo cancellers, one associated with each ofreceiver, each echo canceller for providing a replica echo impairmentsignal, a plurality of second devices, one associated with eachreceiver, each second device responsive to the output of the equalizersystem associated with such receiver and the replica echo impairmentsignal provided by the echo cancellation system associated with suchreceiver for substantially removing the echo impairment signals from thecombination signal and providing the signal to the delay device and thesecond detector.
 38. A method for reducing noise in a communicationssystem comprising a communication line having a plurality of twistedwire pairs, a plurality of transmitters, one transmitter at each end ofeach of the twisted wire pairs, a plurality of receivers, one receiverat each end of each of the twisted wire pairs, each receiver receiving acombination signal including a direct signal from the transmitter at theopposite end of the twisted wire pair with which the receiver isassociated and a plurality of far-end crosstalk (FEXT) impairmentsignals, one from each of the remaining transmitters at the opposite endof the communications line; said method comprising, for each receiver,the steps of: generating a replica FEXT impairment signal; and combiningthe replica FEXT impairment signal with the combination signal toproduce an output signal substantially devoid of FEXT impairmentsignals.
 39. The method of claim 38 wherein the step of generating thereplica FEXT impairment signal comprises the steps of: receiving asignal from each receiver, except for the receiver for which the replicaFEXT impairment signal is being generated; generating, based on eachreceived signal, an individual replica FEXT impairment signal; andcombining the individual replica FEXT impairment signals to form thereplica FEXT impairment signal.
 40. The method of claim 38 wherein thestep of combining the replica FEXT impairment signal with thecombination signal comprises the step of subtracting the replica FEXTimpairment signal from the combination signal.
 41. The method of claim38 wherein the communications system further comprises a plurality ofcombining devices, one associated with each receiver, each combiningdevice responsive to the combination signal and the replica FEXTimpairment signal, wherein, when the direct signal arrives at thereceiver after the FEXT impairment signals, the step of combining thereplica FEXT impairment signal with the combination signal comprises thesteps of: delaying the arrival of the combination signal, at thecombining device, by an amount substantially equal to the time delaybetween the arrival, at the receiver, of the FEXT impairment signals andthe direct signal; and subtracting the replica FEXT impairment signalfrom the combination signal.
 42. The method of claim 38 wherein thecommunications system further comprises a plurality of combiningdevices, one associated with each receiver, each combining deviceresponsive to the combination signal and the replica FEXT impairmentsignal, wherein, when the direct signal arrives at the receiver afterthe FEXT impairment signals, the step of combining the replica FEXTimpairment signal with the combination signal comprises the steps of:delaying the arrival of the combination signal, at the combining device,by an amount greater than the time delay between the arrival, at thereceiver, of the FEXT impairment signals and the direct signal; delayingthe arrival of the replica FEXT impairment signal, at the combiningdevice, by an amount substantially equal to the greater amount; andsubtracting the replica FEXT impairment signal from the combinationsignal.
 43. The method of claim 38 wherein the communications systemfurther comprises a plurality of combining devices, one associated witheach receiver, each combining device responsive to the combinationsignal and the replica FEXT impairment signal, wherein, when the FEXTimpairment signals arrive at the receiver after the direct signal, thestep of combining the replica FEXT impairment signal with thecombination signal comprises the steps of: delaying the arrival of thereplica FEXT impairment, at the combining device, by an amountsubstantially equal to the time delay between the arrival, at thereceiver, of the direct signal and the FEXT impairment signals; andsubtracting the replica FEXT impairment signal from the combinationsignal.
 44. The method of claim 38 wherein the combination signalfurther includes a plurality of near-end crosstalk (NEXT) impairmentsignals, one from each of the transmitters located at the same end ofthe communications line, except for the transmitter at the same end ofthe twisted wire pair with which the receiver is associated; said methodfurther comprising the steps of: generating a replica NEXT impairmentsignal; and combining the replica NEXT impairment signal with thecombination signal to produce an output signal substantially devoid ofNEXT impairment signals
 45. The method of claim 38 wherein thecombination signal further includes an echo impairment signal receivedfrom the transmitter at the same end of the twisted wire pair with whichthe receiver is associated; said method further comprising the steps of:generating a replica echo impairment signal; and combining the replicaecho impairment signal with the combination signal to produce an outputsignal substantially devoid of echo impairment signals.
 46. The methodof claim 38 wherein the combination signal further includes a pluralityof near-end crosstalk (NEXT) impairment signals, one from each of thetransmitters located at the same end of the communications line, exceptfor the transmitter at the same end of the twisted wire pair with whichthe receiver is associated, and an echo impairment signal received fromthe transmitter at the same end of the twisted wire pair with which thereceiver is associated; said method further comprising the steps of:generating a replica NEXT impairment signal; generating a replica echoimpairment signal; and combining the replica NEXT impairment signal andthe replica echo impairment signal with the combination signal toproduce an output signal substantially devoid of noise.
 47. A method forreducing noise in a communications system comprising a communicationline having a plurality of twisted wire pairs, a plurality oftransmitters, one transmitter at each end of each twisted wire pair, aplurality of receivers, one receiver at each end of each twisted wirepair, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of noise signalsincluding far-end crosstalk (FEXT), near-end crosstalk (NEXT), and echoimpairment signals, said method comprising the steps of: generating areplica FEXT impairment signal; generating a replica NEXT impairmentsignal; generating a replica echo impairment signal; and combining eachreplica impairment signal with the combination signal to produce anoutput signal substantially devoid of noise.
 48. The method of claim 47wherein the communications system further comprises a plurality ofsecond combining devices, one associated with each receiver, each secondcombining device responsive to the combination signal and the replicaNEXT and echo impairment signals, and a plurality of first combiningdevices, one associated with each receiver, each first combining deviceresponsive to the output of the second combining device and the replicaFEXT impairment signals, wherein, when the direct signal arrives at thereceiver after the FEXT impairment signals, the step of combining thereplica impairment signal with the combination signal comprises thesteps of: subtracting, at the second combining device, the NEXT and echoimpairment signals from the combination signal; delaying the arrival ofthe output of the second combining device, at the first combiningdevice, by an amount substantially equal to the time delay between thearrival, at the receiver, of the FEXT impairment signals and the directsignal; and subtracting the replica FEXT impairment signal from theoutput of the second combining device.
 49. The method of claim 47wherein the communications system further comprises a plurality ofsecond combining devices, one associated with each receiver, each secondcombining device responsive to the combination signal and the replicaNEXT and echo impairment signals and a plurality of first combiningdevices, one associated with each receiver, each first combining deviceresponsive to the output of the second combining device and the replicaFEXT impairment signals, wherein, when the direct signal arrives at thereceiver after the FEXT impairment signals, the step of combining thereplica impairment signals with the combination signal comprises thesteps of: subtracting, at the second combining device, the NEXT and echoimpairment signals from the combination signal; delaying the arrival ofthe combination signal, at the first combining device, by an amountgreater than the time delay between the arrival, at the receiver, of theFEXT impairment signals and the direct signal; delaying the arrival ofthe replica FEXT impairment signal, at the first combining device, by anamount substantially equal to the greater amount; and subtracting thereplica FEXT impairment signal from the output of the second combiningdevice.
 50. The method of claim 47 wherein the communications systemfurther comprises a plurality of second combining devices, oneassociated with each receiver, each second combining device responsiveto the combination signal and the replica NEXT and echo impairmentsignals and a plurality of first combining devices, one associated witheach receiver, each first combining device responsive to the output ofthe second combining device and the replica FEXT impairment signals,wherein, when the FEXT impairment signals arrive at the receiver afterthe direct signal, the step of combining the replica impairment signalswith the combination signal comprises the steps of: subtracting, at thesecond combining device, the NEXT and echo impairment signals from thecombination signal; delaying the arrival of the FEXT impairment signal,at the first combining device, by an amount substantially equal to thetime delay between the arrival, at the receiver, of the direct signaland the FEXT impairment signals; and subtracting the replica FEXTimpairment signal from the output of the second combining device.
 51. Anoise reduction system for use in a communications system comprising acommunication line having a plurality of twisted wire pairs, a pluralityof transmitters, one transmitter at each end of each twisted wire pair,a plurality of receivers, one receiver at each end of each twisted wirepair, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of noise signalsincluding far-end crosstalk (FEXT), near-end crosstalk (NEXT), and echoimpairment signals, said noise reduction system comprising: a pluralityof NEXT cancellation systems, one associated with each receiver, saidNEXT cancellation system including at least one NEXT canceller, saidNEXT cancellation system responsive to the transmitted signals from thetransmitters at the same end of the communications line as the receiverwith which the NEXT cancellation system is associated, except for thetransmitter at the same end of the twisted wire pair as the receiverwith which the NEXT cancellation system is associated, the NEXTcancellation system responsive to the transmitted signals for generatinga replica NEXT impairment signal; and a plurality of devices, oneassociated with each receiver, said device responsive to the combinationsignal and the replica NEXT impairment signal for providing an outputsignal substantially devoid of NEXT impairment signals.
 52. The noisereduction system of claim 51 further comprising a plurality of FEXTcancellation systems, one associated with each receiver, said FEXTcancellation system responsive to the transmitted signals from thetransmitters at the opposite end of the communications line as thereceiver with which the FEXT cancellation system is associated, exceptfor the transmitter at the opposite end of the twisted wire pair as thereceiver with which the FEXT cancellation system is associated, the FEXTcancellation system responsive to the transmitted signals for generatinga replica FEXT impairment signal; wherein said device is furtherresponsive to the replica FEXT impairment signal for providing an outputsignal substantially devoid of FEXT impairment signals.
 53. The noisereduction system of claim 51 further comprising a plurality of skewcompensators, one associated with each receiver, said skew compensatorresponsive to the output of the device associated with such receiver,said skew compensator for adjusting the delay of the output signal fromsuch device so that such delayed output signal is in sync with otheroutput signals from the other devices.
 54. The noise reduction system ofclaim 53 further comprising a plurality of FEXT cancellation systems,one associated with each receiver, said FEXT cancellation systemresponsive to the transmitted signals from the transmitters at theopposite end of the communications line as the receiver with which theFEXT cancellation system is associated, except for the transmitter atthe opposite end of the twisted wire pair as the receiver with which theFEXT cancellation system is associated, the FEXT cancellation systemresponsive to the transmitted signals for generating a replica FEXTimpairment signal; wherein said device is further responsive to thereplica FEXT impairment signal for providing an output signalsubstantially devoid of FEXT impairment signals.
 55. The noise reductionsystem of claim 51 further comprising a decoder responsive to thesequences of the output signals from each of the receivers for passingonly acceptable sequences.
 56. The noise reduction system of claim 55further comprising a plurality of FEXT cancellation systems, oneassociated with each receiver, said FEXT cancellation system responsiveto the transmitted signals from the transmitters at the opposite end ofthe communications line as the receiver with which the FEXT cancellationsystem is associated, except for the transmitter at the opposite end ofthe twisted wire pair as the receiver with which the FEXT cancellationsystem is associated, the FEXT cancellation system responsive to thetransmitted signals for generating a replica FEXT impairment signals;wherein said device is further responsive to the replica FEXT impairmentsignal for providing an output signal substantially devoid of FEXTimpairment signals.
 57. The noise reduction system of claim 51 furthercomprising a decoder responsive to the sequences of the output signalsfrom each of the receivers for (1) passing only acceptable sequences ofsuch signals, (2) discarding unacceptable sequences of such signals and(3) changing sequences of signals with minimal numbers of errors tosequences of signals in which the errors have been corrected to provideacceptable sequences closest to the sequences with the minimal number oferrors.
 58. The noise reduction system of claim 51 further comprising: aplurality of skew compensators, one associated with each receiver, saidskew compensator responsive to the output of the device associated withsuch receiver, said skew compensator for adjusting the delay of theoutput signal from such device so that such delayed output signal is insync with other output signals from the other devices; and a decoderresponsive to the sequences of the output signals from each of the skewcompensators for passing only acceptable sequences of the outputsequences.
 59. The noise reduction system of claim 58 further comprisinga plurality of FEXT cancellation systems, one associated with eachreceiver, said FEXT cancellation system responsive to the transmittedsignals from the transmitters at the opposite end of the communicationsline as the receiver with which the FEXT cancellation system isassociated, except for the transmitter at the opposite end of thetwisted wire pair as the receiver with which the FEXT cancellationsystem is associated, the FEXT cancellation system responsive to thetransmitted signals for generating a replica FEXT impairment signal;wherein said device is further responsive to the replica FEXT impairmentsignal for providing an output signal substantially devoid of FEXTimpairment signals.
 60. A noise reduction system for use in acommunications system comprising a communication line having a pluralityof twisted wire pairs, a plurality of transmitters, one transmitter ateach end of each twisted wire pair, a plurality of receivers, onereceiver at each end of each twisted wire pair, each receiver receivinga combination signal including a direct signal from the transmitter atthe opposite end of the twisted wire pair with which the receiver isassociated and a plurality of noise signals including far-end crosstalk(FEXT), near-end crosstalk (NEXT), and echo impairment signals, saidnoise reduction system comprising: a plurality of echo cancellers, atleast one of said echo cancellers associated with each receiver, saidecho canceller for receiving the transmitted signals from thetransmitter at the same end of the twisted wire pair as that of thereceiver with which the echo canceller is associated, each echocanceller responsive to the transmitted signal for generating a replicaof the echo impairment signal; and a plurality of devices, oneassociated with each receiver, said device responsive to the combinationsignal and the replica echo impairment signal for providing an outputsignal substantially devoid of noise.
 61. The noise reduction system ofclaim 60 further comprising a plurality of FEXT cancellation systems,one associated with each receiver, said FEXT cancellation systemresponsive to the transmitted signals from the transmitters at theopposite end of the communications line as the receiver with which theFEXT cancellation system is associated, except for the transmitter atthe opposite end of the twisted wire pair as the receiver with which theFEXT cancellation system is associated, the FEXT cancellation systemresponsive to the transmitted signals for generating a replica FEXTimpairment signal; wherein said device is further responsive to thereplica FEXT impairment signal for providing an output signalsubstantially devoid of FEXT impairment signals.
 62. The noise reductionsystem of claim 60 further comprising a plurality of skew compensators,one associated with each receiver, said skew compensator responsive tothe output of the device associated with such receiver, said skewcompensator for adjusting the delay of the output signal from suchdevice so that such delayed output signal is in sync with other outputsignals from the other devices.
 63. The noise reduction system of claim62 further comprising a plurality of FEXT cancellation systems, oneassociated with each receiver, said FEXT cancellation system responsiveto the transmitted signals from the transmitters at the opposite end ofthe communications line as the receiver with which the FEXT cancellationsystem is associated, except for the transmitter at the opposite end ofthe twisted wire pair as the receiver with which the FEXT cancellationsystem is associated, the FEXT cancellation system responsive to thetransmitted signals for generating a replica FEXT impairment signal;wherein said device is further responsive to the replica FEXT impairmentsignal for providing an output signal substantially devoid of FEXTimpairment signals.
 64. The noise reduction system of claim furthercomprising a decoder responsive to the sequences of the output signalsfrom each of the receivers for passing only acceptable sequences. 65.The noise reduction system of claim 64 further comprising a plurality ofFEXT cancellation systems, one associated with the receiver, said FEXTcancellation system responsive to the transmitted signals from thetransmitters at the opposite end of the communications line as thereceiver with which the FEXT cancellation system is associated, exceptfor the transmitter at the opposite end of the twisted wire pair as thereceiver with which the FEXT cancellation system is associated, the FEXTcancellation system responsive to the transmitted signals for generatinga replica FEXT impairment signal; wherein said device is furtherresponsive to the replica FEXT impairment signal for providing an outputsignal substantially devoid of FEXT impairment signals.
 66. The noisereduction system of claim 60 further comprising a decoder responsive tothe sequences of the output signals from each of the receivers for (1)passing only acceptable sequences of such signals, (2) discardingunacceptable sequences of such signals and (3) changing sequences ofsignals with minimal numbers of errors to sequences of signals in whichthe errors have been corrected to provide acceptable sequences closestto the sequences with the minimal number of errors.
 67. The noisereduction system of claim 60 further comprising: a plurality of skewcompensators, one associated with each receiver, said skew compensatorresponsive to the output of the device associated with such receiver,said skew compensator for adjusting the delay of the output signal fromsuch device so that such delayed output signal is in sync with otheroutput signals from the other devices; and a decoder responsive to thesequences of the output signals from each of the skew compensators forpassing only acceptable sequences of the output sequences.
 68. The noisereduction system of claim 67 further comprising a plurality of FEXTcancellation systems, one associated with each receiver, said FEXTcancellation system responsive to the transmitted signals from thetransmitters positioned at the opposite end of the communications lineas the receiver with which the FEXT cancellation system is associated,except for the transmitter at the opposite end of the twisted wire pairas the receiver with which the FEXT cancellation system is associated,the FEXT cancellation system responsive to the transmitted signals forgenerating a replica FEXT impairment signal; wherein said device isfurther responsive to the replica FEXT impairment signal for providingan output signal substantially devoid of FEXT impairment signals.
 69. Anoise reduction system for use in a communications system comprising acommunication line having a plurality of twisted wire pairs, a pluralityof transmitters, one transmitter at each end of each twisted wire pair,a plurality of receivers, one receiver at each end of each twisted wirepair, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of noise signalsincluding far-end crosstalk (FEXT), near-end crosstalk (NEXT), and echoimpairment signals, said noise reduction system comprising: a pluralityof devices, one associated with each receiver, said device responsive tothe combination signal for providing an output signal; a plurality ofskew compensators, one associated with each receiver, said skewcompensator responsive to the output of the device associated with suchreceiver, said skew compensator for adjusting the delay of the outputsignal from such device so that such delayed output signal is in syncwith other output signals from the other devices.
 70. The noisereduction system of claim 69 further comprising a plurality of FEXTcancellation systems, one associated with each receiver, said FEXTcancellation system responsive to the transmitted signals from thetransmitters at the opposite end of the communications line as thereceiver with which the FEXT cancellation system is associated, exceptfor the transmitter at the opposite end of the twisted wire pair as thereceiver with which the FEXT cancellation system is associated, the FEXTcancellation system responsive to the transmitted signals for generatinga replica FEXT impairment signal; wherein said device is furtherresponsive to the replica FEXT impairment signal for providing an outputsignal substantially devoid of FEXT impairment signals.
 71. The noisereduction system of claim 69 further comprising a decoder responsive tothe sequences of the output signals from each of the receivers forpassing only acceptable sequences.
 72. The noise reduction system ofclaim 71 further comprising a plurality of FEXT cancellation systems,one associated with each receiver, said FEXT cancellation systemresponsive to the transmitted signals from the transmitters at theopposite end of the communications line as the receiver with which theFEXT cancellation system is associated, except for the transmitter atthe opposite end of the twisted wire pair as the receiver with which theFEXT cancellation system is associated, the FEXT cancellation systemresponsive to the transmitted signals for generating a replica FEXTimpairment signal; wherein said device is further responsive to thereplica FEXT impairment signal for providing an output signalsubstantially devoid of FEXT impairment signals.
 73. The noise reductionsystem of claim 69 further comprising a decoder responsive to thesequences of the output signals from each of the receivers for (1)passing only acceptable sequences of such signals, (2) discardingunacceptable sequences of such signals and (3) changing sequences ofsignals with minimal numbers of errors to sequences of signals in whichthe errors have been corrected to provide acceptable sequences closestto the sequences with the minimal number of errors.
 74. A noisereduction system for use in a communications system comprising acommunication line having a plurality of twisted wire pairs, a pluralityof transmitters, one transmitter at each end of each twisted wire pair,a plurality of receivers, one receiver at each end of each twisted wirepair, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of noise signalsincluding far-end crosstalk (FEXT), near-end crosstalk (NEXT), and echoimpairment signals, said noise reduction system comprising: a pluralityof NEXT cancellation systems, one associated with each receiver, eachNEXT cancellation system for receiving the transmitted signals from thetransmitters at the same end of the communications line as the receiverwith which the NEXT cancellation system is associated, except for thetransmitter at the same end of the twisted wire pair as the receiverwith which the NEXT cancellation system is associated, each NEXTcancellation system responsive to the transmitted signals for generatinga replica NEXT impairment signal; a plurality of echo cancellers, atleast one of said echo cancellers associated with each receiver, eachecho canceller for receiving the transmitted signals from thetransmitter at the same end of the twisted wire pair as that of thereceiver with which the echo canceller is associated, each echocanceller responsive to the transmitted signal for generating a replicaecho impairment signal; a plurality of FEXT cancellation systems, oneassociated with each receiver, each FEXT cancellation system forreceiving the transmitted signals from the transmitters at the oppositeend of the communications line as the receiver with which the FEXTcancellation system is associated, except for the transmitter at theopposite end of the twisted wire pair as the receiver with which theFEXT cancellation system is associated, each FEXT cancellation systemresponsive to the transmitted signals for generating a replica FEXTimpairment signal; a plurality of first summing devices, one associatedwith each receiver, said first summing device responsive to thecombination signal, the replica NEXT impairment signal and the replicaecho impairment signal for providing an initial output signalsubstantially devoid of NEXT impairment signals and echo impairmentsignals; a plurality of skew compensators, one associated with eachreceiver, said skew compensator responsive to the output of the firstsumming device associated with such receiver, said skew compensator foradjusting the delay of the initial output signal from such summingdevice so that such delayed initial output signal is in sync with otheroutput signals from the other first summing devices; and a plurality ofsecond summing devices, one associated with each receiver, said secondsumming device responsive to the delayed initial output signal and thereplica FEXT impairment signal for providing a final output signalsubstantially devoid of FEXT impairment signal.
 75. The noise reductionsystem of claim 74 further comprising a decoder responsive to thesequences of the final output signals from each of the skew compensatorsfor passing only acceptable sequences.
 76. A method for reducing noisein a communications system comprising a communication line having aplurality of twisted wire pairs, a plurality of transmitters, onetransmitter at each end of each of the twisted wire pairs, a pluralityof receivers, one receiver at each end of each of the twisted wirepairs, each receiver receiving a combination signal including a directsignal from the transmitter at the opposite end of the twisted wire pairwith which the receiver is associated and a plurality of noise signalsincluding far-end crosstalk (FEXT), near-end crosstalk (NEXT), and echoimpairment signals; said method comprising, for each receiver, the stepsof: generating a replica FEXT impairment signal; and combining thereplica FEXT impairment signal with the combination signal to produce anoutput signal substantially devoid of FEXT impairment signals. delayingthe output signal from such receiver so that such delayed output signalis in sync with other output signals from the other receivers.
 77. Themethod of claim 76 further comprising the steps of: monitoring asequence of signals; passing only acceptable sequences of such signals;discarding unacceptable sequences of such signals; changing sequences ofsignals with minimal numbers of errors to sequences of signals in whichthe errors have been corrected to provide acceptable sequences closestto the sequences with the minimal number of errors.
 78. The method ofclaim 77 wherein the output signal is in the form of symbols, eachcomprising a plurality of bits, and the sequence is a series of symbolsfrom an individual receiver.
 79. The method of claim 77 wherein theoutput signal is in the form of symbols, each comprising a plurality ofbits, and the sequence is a set of symbols comprising at least onesymbol from each of the receivers.
 80. In combination, a communicationssystem comprising a communications line having a plurality of twistedwire pairs and having a transmitter and a receiver connected at each endof each twisted wire pair wherein far-end crosstalk (FEXT) is generatedat each receiver from noise passing to such receiver from the ends ofthe twisted line pairs opposite the receiver, other than from theopposite end of the twisted line pair connected to the receiver, and aplurality of systems each associated with the receiver connected to thetwisted line pair at one end of the twisted line pair for eliminating,from the signals at the receiver, the FEXT at the receiver from theother end of the other twisted wire pairs.
 81. In a combination as setforth in claim 80 noise being generated at each receiver from the otherend of the twisted wire pair connected to the receiver, each of thesystems in the plurality being operative to eliminate, from the signalsat the receiver at the one end of the twisted wire pair, the noisegenerated at the receiver from the other end of the twisted wire pair.82. In a combination as set forth in claim 80 noise being generated ateach receiver, from the ends of the twisted wire pairs adjacent to thereceiver other than the twisted wire pair connected at the other end ofthe receiver, each of the systems in the plurality being operative toeliminate, from the signals at the receiver connected at the one end ofthe twisted wire pair, the noise generated at the receiver from the endsof the twisted wires adjacent to the receiver other than the twistedwire pair connected at the one end of the receiver.
 83. In a combinationas set forth in claim 80 a decoder responsive to the sequences of thesignals from the receivers at the one end of the twisted wire pairs fordetermining whether such sequences of signals are acceptable and forprocessing such sequences of signals in accordance with suchdeterminations.
 84. In a combination as set forth in claim 80, noisebeing generated at each receiver from the other end of the twisted wirepair connected at the one end to the receiver and noise being generatedat each receiver from the ends of the twisted wire pairs adjacent to thereceiver other than the twisted wire pair connected to the receiver,each of the systems in the plurality being operative to eliminate, fromthe signals at the receivers connected at the one end of the twistedwire pair, the noise generated at the receiver from the other end of thetwisted wire pair, each of the systems in the plurality being operativeto eliminate, from the signals at the receiver connected at the one endto the twisted wire pair, the noise generated at the receiver from theends of the twisted wires adjacent to the receiver other than thetwisted wire pair connected at the one end to the receiver, and adecoder responsive to the sequences of the signals from the receivers atthe same ends of the twisted pairs for determining whether suchsequences are acceptable and for processing such sequences of signals inaccordance with such determinations.
 85. In a combination as set forthin claim 80, a skew compensator associated with each receiver andresponsive to the signals from the receiver for delaying such signals sothat such delayed signals are in sync with the delayed signals from theother receivers before the signals are introduced to the systems in theplurality.
 86. In a combination as set forth in claim 84, a skewcompensator associated with each receiver and responsive to the signalsfrom the receiver for delaying such signals so that such delayed signalsare in sync with the delayed signals from the other receivers before thesignals are introduced to the systems in the plurality.
 87. Incombination, a system comprising a communication line having a pluralityof twisted wire pairs and having a transmitter and a receiver connectedat each end of each twisted wire pair to transmit and receive sequencesof signals, and a decoder responsive to the sequences of the signalsfrom the receivers at the same ends of the twisted wire pairs fordetermining whether such sequences of signals are acceptable and forprocessing such sequences of signals in accordance with suchdeterminations.
 88. In a combination as set forth in claim 87 whereinthe decoder determines whether the sequences of the signals at thereceivers are acceptable or unacceptable and if unacceptable, whetherthe unacceptability is minor or major and wherein the decoder passes theacceptable sequences of signals, the decoder converts the unacceptablesequences of signals to acceptable sequences of signals when theunacceptability is minor and the decoder rejects the unacceptablesequences of signals when the unacceptability is major.
 89. In acombination as set forth in claim 87 wherein the signals in thesequences at the receivers include noise and wherein the system removesthe noise from the signals in the sequences before the sequences ofsignals are introduced to the decoder.
 90. In a combination as set forthin claim 87 wherein the signals in the sequences at each of thereceivers include noise passing to the receiver from the other ends ofthe twisted line pairs other than from the other end of the twisted linepair connected to the receiver and wherein the system is operative toeliminate such noise from the signals at each receiver before thesignals pass to the decoder.
 91. In a combination as set forth in claim87 wherein the signals in the sequences at each of the receivers includenoise generated at the receiver from the other end of the twisted wirepair connected to the receiver and wherein the system is operative toeliminate, from the signals at the receiver connected to the twistedwire pair, the noise generated at the receiver from the other end of thetwisted wire pair.
 92. In a combination as set forth in claim 87 whereinthe signals in the sequences at each of the receivers include noisegenerated at each receiver from the ends of the twisted wire pairsadjacent to the receiver other than from the twisted wire pair connectedto the receiver, the system being operative to eliminate, from thesignals at the receiver connected to the twisted wire pair, the noisegenerated at the receiver from the ends of the twisted wire pairsadjacent to the receiver other than from the twisted wire pair connectedto the receiver.
 93. In a combination as set forth in claim 88 thesignals in the sequences at each of the receivers include noise passingto the receiver from the other ends of the twisted line pairs other thanfrom the other end of the twisted line pair connected to the receiverand wherein the system is operative to eliminate such noise from thesignals at each receiver before the signals pass to the decoder andwherein, the signals in the sequences at each of the receivers includenoise generated at the receiver from the other end of the twisted wirepair connected to the receiver and wherein the system is operative toeliminate, from the signals at the receiver connected to the twistedwire pair, the noise generated at the receiver from the other end of thetwisted wire pair and wherein, the signals in the sequences at each ofthe receivers include noise generated at each receiver from the ends ofthe twisted wire pairs adjacent to the receiver other than from thetwisted wire pair connected to the receiver and wherein the systemoperative to eliminate, from the signals at the receiver connected tothe twisted wire pair, the noise generated at the receiver from the endsof the twisted wire pairs adjacent to the receiver other than from thetwisted wire pair connected to the receiver.
 94. In a combination as setforth in claim 87, a compensator responsive to the signals in thesequences from the receivers for adjusting the times of such signals toprovide for the introduction of such signals to the decoder in timesynchronization.